All animal embryos, which begin as a disc or sphere, form an elongated body axis with a head on one end and a tail on the other. This fundamental process of axial elongation is accomplished by convergence and extension (C&E) of embryonic tissues, a process driven by polarized cell behaviors such as cell migration and mediolateral (ML) intercalation during gastrulation. Although molecular regulation of ML cell polarity in embryonic tissues is largely attributed to planar cell polarity (PCP) signaling, there is evidence that signals from the notochord-somite boundary (NSB) are also involved. The nature of this NSB signal and its relationship to PCP signaling are not understood. The Solnica-Krezel lab has identified the chromatin factor Ugly duckling (Udu)/Gon4l as a novel regulator of C&E in zebrafish. Maternal zygotic (MZ) udu mutant embryos display an extremely short anterior-posterior axis, and interestingly, abnormal NSB and reduced ML polarity of boundary-adjacent cells. Complete loss of udu/gon4l function enhances axis elongation defects in PCP mutant embryos, and PCP-dependent cell polarity appears to be unaffected in MZudu embryos, suggesting that udu/gon4l regulates ML cell polarity in parallel to the PCP pathway. We found that expression of slit3, which encodes a repulsive ECM ligand, is increased in MZudu embryos. Expression of Slit3 and its Robo receptors in the axial mesoderm of WT embryos predict a role for Slit-Robo signaling in formation of normal axial mesoderm and the NSB. Furthermore, overexpression of slit3 disrupts NSB formation and C&E in zebrafish embryos, and exacerbates axis elongation defects in PCP mutants. I hypothesize that udu/gon4l is required for intact Slit-Robo signaling at the NSB, which mediates a novel polarity cue that acts in parallel to PCP to regulate ML cell polarity underlying C&E. Characterization of this novel polarity cue, and its relationship to known polarity signals, will further our understanding of how complex positional and morphogenetic cues are coordinated during vertebrate embryogenesis.